Many non-volatile semiconductor memories, such as EPROMs, rely on so-called "hot electrons" and other phenomena associated with very small geometry MOS devices to provide the mechanism by which charge is accumulated on the floating gates of the transistors in the memory array. Therefore, any process for fabricating such memories must retain, at least to some degree, the desired phenomena.
However, any non-volatile memory device must include, in addition to the array of memory cells, certain peripheral circuits including address decoders, sense amplifiers and other logic. The small geometry-related phenomena upon which the memory cells rely adversely effect the transistors comprising the peripheral circuits and require the use of various solutions which typically sacrifice die area or performance.
Several techniques are in common use which provide relief from certain small geometry-related phenomena. For instance, techniques referred to variously as "low dose drain", "lightly-doped drain" and "graded source/drain" all involve some modification to the lateral and possibly the vertical dopant profile of the source/drain regions. Of particular interest in the case of peripheral circuits on a nonvolatile memory are lateral profile modifications which provide relief from short-channel induced hot electron effects.
Up to now, however, it has not been shown that one can successfully merge the modified source/drain techniques with a process capable of fabricating useful non-volatile memories in such a way that the peripheral circuits are provided the benefits of the modified regions while preserving the functionality of the memory cells without incurring the penalty of extra critical mask steps which complicate the process.
Throughout the following description, it will be understood that when a material is referred to as "poly", the material may be poly-crystalline silicon or a multilayer construct including both a metal silicide such as tungsten silicide and poly-crystalline silicon. In addition, it will be understood that two distinct types of modified source/drain processes will be referred to as "low dose drain" (LDD) and "graded source/drain" (GSD), respectively. LDD processes are characterized by an initial, grading implant followed by the formation of a spacer followed by a second, higher dose implant. GSD processes are characterized by the formation of a spacer, followed by a grading implant with a fast-diffusing species and a source/drain implant with a slow-diffusing species. While the usage of these terms is not well settled in the art, this usage will be followed for present purposes. Both processes, and other similar processes, will be referred to as modified source/drain processes.